Whatever their sources of information—whether Chevreul, or Charles Blanc, or Rood, Helmholtz, or even Goethe—painters had to confront a particular fact: the physiological screen through which light passes to the human brain is not transparent, like a window pane; it is, like a filter, involved in a set of specific distortions. For us, as human perceivers, there is an unbreachable gulf between "real" color and "seen" color. We may be able to measure the first; but we can only experience the second. And this is because, among other things, color is always involved in interaction-one color reading onto and affecting its neighbor. Even if we are only looking at a single color, there is still interaction, because the retinal excitation of the afterimage will superimpose on the first chromatic stimulus that of a second, which is its complementary. The whole issue of complementary colors, along with the whole edifice of color harmonics that painters constructed on its basis, was thus a matter of physiological optics.

An interesting feature of treatises written on physiological optics is that they were illustrated with grids. Because it was a matter of demonstrating the interaction of specific particles throughout a continuous field, that field was analyzed into the modular and repetitive structure of the grid. So for the artist who wished to enlarge his understanding of vision in the direction of science, the grid was there as a matrix of knowledge. By its very abstraction, the grid conveyed one of the basic laws of knowledge-the separation of the perceptual screen from that of the "real" world. Given all of this, it is not surprising that the grid-as an emblem of the infrastructure of vision-should become an increasingly insistent and visible feature of neo-impressionist painting, as Seurat, Signac, Cross, and Luce applied themselves to the lessons of physiological optics. Just as it is not surprising that the more they applied these lessons, the more "abstract" their art became, so that as the critic Felix Feneon observed of the work of Seurat, science began to yield its opposite, which is symbolism.